Double stranded DNA (deoxyribonucleic acid) and DNA/RNA (ribonucleic acid) and RNA/RNA complexes in the familiar double helical configuration are stable molecules that, in vitro, require aggressive conditions to separate the complementary strands of the nucleic acid. Known methods that are commonly employed for strand separation require the use of high temperatures of at least 60.degree. celsius and often 100.degree. celsius for extended periods of ten minutes or more or use an alkaline pH of 11 or higher. Other methods include the use of helicase enzymes such as Rep protein of E. coli that can catalyse the unwinding of the DNA in an unknown way, or binding proteins such as 32-protein of E.coli phage T4 that act to stabilise the single stranded form of DNA. The denatured single stranded DNA produced by the known processes of heat or alkali is used commonly for hybridisation studies or is subjected to amplification cycles.
U.S. Pat. No. 4,683,202 (Kary B Mullis et al, assigned to Cetus Corporation) discloses a process for amplifying and detecting a target nucleic acid sequence contained in a nucleic acid or mixture thereof by separating the complementary strands of the nucleic acid, hybridising with specific oligonucleotide primers, extending the primers with a polymerase to form complementary primer extension products and then using those extension products for the further synthesis of the desired nucleic acid sequence by allowing hybridisation with the specific oligonucleotides primers to take place again. The process can be carried out repetitively to generate large quantities of the required nucleic acid sequence from even a single molecule of the starting material. Separation of the complementary strands of the nucleic acid is achieved preferably by thermal denaturation in successive cycles, since only the thermal process offers simple reversibility of the denaturation process to reform the double stranded nucleic acid, in order to continue the amplification cycle. However the need for thermal cycling of the reaction mixture limits the speed at which the multiplication process can be carried out owing to the slowness of typical heating and cooling systems. It also requires the use of special heat resistant polymerase enzymes from thermophilic organisms for the primer extension step if the continuous addition of heat labile enzyme is to be avoided. It limits the design of new diagnostic formats that use the amplification process because heat is difficult to apply in selective regions of a diagnostic device and it also can be destructive to the structure of the DNA itself because the phosphodiester bonds may be broken at high temperatures leading to a collection of broken single strands. It is generally believed that the thermophilic polymerases in use today have a lower fidelity ie. make more errors in copying DNA than do enzymes from mesophiles. It is also the case that thermophilic enzymes such as TAQ polymerase have a lower turnover number than heat labile enzymes such as the Klenow polymerase from E.coli. In addition, the need to heat to high temperatures, usually 90.degree. celsius or higher to denature the nucleic acid leads to complications when small volumes are used as the evaporation of the liquid is difficult to control. These limitations have so far placed some restrictions on the use of the Mullis et al process in applications requiring very low reagent volumes to provide reagent economy, in applications where the greatest accuracy of copy is required such as in the Human Genome sequencing project and in the routine diagnostics industry where reagent economy, the design of the assay format and the speed of the DNA denaturation/renaturation process are important.
Denaturation/renaturation cycles are also required in order to perform the so-called ligase chain reaction described in EP-A-0320308 in which amplification is obtained by ligation of primers hybridised to template sequences rather than by extending them.
It is known that DNA has electrochemical properties. For example, N. L. Palacek (in "Electrochemical Behaviour of Biological Macromolecules", Bioelectrochemistry and Bioenergetics, 15, (1986), 275-295) discloses the electrochemical reduction of adenine and cytosine in thermally denatured single stranded DNA at about -(minus) 1.5 V on the surface of a mercury electrode. This reduction process also requires a prior protonation and therefore takes place at a pH below 7.0. The primary reduction sites of adenine and cytosine form part of the hydrogen bonds in the Watson-Crick base pairs. Palacek was unable to demonstrate reduction of adenine and cytosine in intact, native double stranded DNA at the mercury electrode. Palacek has further demonstrated that to a very limited extent the DNA double helix is opened on the surface of the mercury electrode at a narrow range of potentials centred at -(minus)1.2 V in a slow process involving an appreciable part of the DNA molecule. This change in the helical structure of the DNA is thought to be due to prolonged interaction with the electrode charged to certain potentials and is not thought to be a process involving electron transfer to the DNA. No accumulation of single stranded DNA in the working solution was obtained and no practical utility for the phenomenon was suggested. Palacek also reports that the guanine residues in DNA can be reduced at -(minus)1.8 V to dihydroguanine which can be oxidised back to guanine at around -(minus)0.3 V. The reducible guanine double bond is not part of the hydrogen bonds in the Watson-Crick base pairs and this electrochemical process involving guanine does not affect the structure of the DNA double helix.
In an earlier paper F. Jelen and E. Palacek (in "Nucleotide Sequence-Dependent Opening of Double-Stranded DNA at an Electrically Charged Surface", Gen. Physiol. Biophys., (1985), 4, pp 219-237), describe in more detail the opening of the DNA double helix on prolonged contact of the DNA molecules with the surface of a mercury electrode. The mechanism of opening of the helix is postulated to be anchoring of the polynucleotide chain via the hydrophobic bases to the electrode surface after which the negatively charged phosphate residues of the DNA are strongly repelled from the electrode surface at an applied potential close to -(minus)1.2 V, the strand separation being brought about as a result of the electric field provided by the cathode. There is no disclosure of separating the strands of the DNA double helix while the DNA is in solution (rather than adsorbed onto the electrode) and there is no disclosure of useful amounts of single strand DNA in solution. Furthermore, there is no disclosure that the nucleotide base sequence of the DNA on the electrode is accessible from solution. The bases themselves are tightly bound to the mercury surface. A mercury electrode is a complex system and the electrode can only be operated in the research laboratory with trained technical staff.
H W Nurnberg ("Applications of Advanced Voltammetric Methods in Electrochemistry" in "Bioelectrochemistry", Plenum Inc (New York), 1983, pp. 183-225) discloses partial helix opening of adsorbed regions of native DNA to a mercury electrode surface to form a so-called ladder structure. However, the DNA is effectively inseparably bound to or adsorbed onto the electrode surface. In this condition, we believe the denatured DNA to be of no use for any subsequent process of amplification or analysis. To be of any use, the denatured DNA must be accessible to subsequent processes and this is conveniently achieved if the single stranded DNA is available in free solution or is associated with the electrode in some way but remains accessible to further processes. Nurnberg has not demonstrated the ability of the mercury electrode to provide useful quantities of single stranded DNA.
V. Brabec and K. Niki ("Raman scattering from nucleic acids adsorbed at a silver electrode" in Biophysical Chemistry (1985), 23, pp 63-70) have provided a useful summary of the differing views from several workers on DNA denaturation at the surface of both mercury and graphite electrodes charged to negative potentials. There has emerged a consensus amongst the research workers in this field that the denaturation process only takes place in DNA that is strongly adsorbed to the electrode surface and only over prolonged periods of treatment with the appropriate negative voltage, a positive voltage having no effect on the double helix.
Brabec and Palacek (J. Electroanal. Chem., 88 (1978) 373-385) disclose that sonicated DNA damaged by gamma radiation is transiently partially denatured on the surface of a mercury pool electrode, the process being detectable by reacting the single stranded products with formaldehyde so as to accumulate methylated DNA products in solution. Intact DNA did not show any observable denaturation.
Our Application No. PCT/GB91/01563 discloses a process for denaturing double-stranded nucleic acid which comprises operating on solution containing nucleic acid with an electrode under conditions such as to convert a substantial portion of said nucleic acid to a wholly or partially single stranded form.
This process was based on a finding that it is possible to produce the denaturation of undamaged (i.e. non-irradiated) DNA at ambient temperature by applying a suitable voltage to a solution in which the DNA is present under suitable conditions.
The mechanism for the process has not yet been fully elucidated. We believe that the process is one in which the electric field at the electrode surface which produces the denaturation of the double helix.
In polymerase chain reaction processes, it has been shown that the denatured DNA produced by the denaturing process is immediately in a suitable state for primer hybridisation and extension. On a larger scale, it has been found that samples of denatured DNA produced either by a negative voltage electrode or thermal denaturation can be caused or encouraged to reanneal by incubation at a higher temperature or by the use of a positive voltage.
Although the process of Application No. PCT/GB91/01563 can take place in a solution containing only the electrode and the nucleic acid dissolved in water containing a suitable buffer, the process can be facilitated by the presence in the solution containing the nucleic acid of a promoter compound. Methyl viologen or a salt thereof was disclosed as the preferred promoter compound.
It is believed that the positively charged viologen molecules interact between the negatively charged DNA and the negatively charged cathode to reduce electrostatic repulsion therebetween and hence to promote the approach of the DNA to the electrode surface where the electrical field is at its strongest. Accordingly, we expressed a preference in Application No. PCT/GB91/01563 to employ as promoters compounds having spaced positively charged centres, e.g. bipolar positively charged compounds. Preferably, the spacing between the positively charged centres was to be similar to that in viologens.